I Have a Big Problem with My Hogwarts

Following last term's haunting, Ravenclaw House has been in big trouble lately.

This morning, a fourth-grade little wizard hurried out of the lounge. After having breakfast in the auditorium, he realized that he had forgotten to bring the book for the first potions class.

Professor Snape is not a kind professor, don't say he will forgive you for forgetting your books, this little wizard has been at Hogwarts for four years, and he has never seen Snape to that student. Laughed, except sneer and sneer.

He could only hurry back to the lounge, still praying in his heart that the riddle of the knocker hadn't been changed, or if he was in a hurry, he could give him a simpler one.

But when he returned to the door of Ravenclaw's lounge, he was dumbfounded to hear today's question.

"Use the ten numbers 0, 1, 2, 3, 4, 5, 6, 7, 8, and 9 to form a five-digit number. How many five-digit numbers are there with three odd numbers and two even numbers? indivual?"

"Here."

The fourth-year little wizard stammered at the knocker on the door that looked like a draught, and immediately wondered if he had heard it wrong.

"Well, please put the question aside."

The Knocker Eagle looked impatient and repeated the question, as if the little wizard had disturbed it.

"What's the problem? Five digits? And what are odd and even numbers?" The little wizard collapsed. "Okay, Brother Eagle, I'll be late if I don't take out the book. Professor Snape will call me. Skin cramps are made into potion materials!"

However, the door knocker eagle ignored his request at all. In the past, there were cases where little wizards encountered urgent matters but could not answer their questions.

The little wizard had no choice. After checking the time, he could only turn around and run towards the classroom.

He will be scolded for not taking the book, but if he is late and has not taken the book, he can hold a death anniversary dinner next year.

After the little wizard who got in the way was fooled away, the door knocker eagle was finally able to continue his investigation.

But it didn't take long for a new group of little wizards to return from the auditorium.

They are Ravenclaw third-year students. Since there is no class this morning, they can go directly to the dormitory after breakfast in the auditorium to spend a leisurely first half day.

"Have you finished the homework assigned by Professor Lockhart?" a third-grade girl smiled and said to the white-haired paralyzed boy beside her.

"finished."

The little witch said in amazement, "So soon, there is a lot of homework in Defense Against the Dark Arts. The professor stipulates that at least fifteen inches of parchment should be filled. How much have you written?"

"18 inches."

The boy's attitude seemed rather cold, but the little wizards and witches around him were not surprised by this. They had been classmates together for three years, and of course they knew that Ian Campbell was like this to everyone.

Although Ian's performance was very cold, the little witch was still very enthusiastic, twittering.

"You always finish your homework first. Who do you do it with? Yesterday, Monica and I wanted to go and do homework with you, but we didn't see you in the lounge."

Ian's indifference is indifference, but he answers any questions he has.

"Lynn from Slytherin and Cedric from Hufflepuff."

"I heard that you and them are friends, and it is said that both of them are top students in their respective colleges, and Lynn Bellock has been the first in our class since the first year."

Hearing what she said, Ian couldn't help thinking that "Lynn"'s recent performance was completely unworthy of his Slytherin's best student, and the title of the first year in the third year.

While they were chatting, a group of people had come to the door of Slytherin's common room. The knocker eagles who were counting saw that they could only stop and asked the question left by Lynn again impatiently. .

"Use the ten numbers 0, 1, 2, 3, 4, 5, 6, 7, 8, and 9 to form a five-digit number. How many five-digit numbers are there with three odd numbers and two even numbers? indivual?"

Ravenclaw's little wizard: ∑(￣□￣;)

"What kind of riddle is this?"

"Why make these numbers into five digits?"

"What is an odd number? What is an even number?"

A group of young wizards became Muggles immediately. They were pure-blooded, mixed-blooded and Muggle families. Pure-blooded and mixed-blooded young wizards basically never went to Muggle primary school, and Muggle families even went to primary school. Not yet to the point of learning this knowledge.

Ian also heard this question. He was born from a noble family, and although he never went to primary school, he had a special tutor at home to teach him since he was a child, so at least he knew what odd and even numbers were.

Looking at the little wizards with no heads around them, he could only explain to them.

"An even number is a number that is divisible by 2. There is a formula for an odd number. Any number obtained from 2n+1 is an odd number."

These little wizards still understand the most basic mathematical knowledge. After all, they are all Ravenclaw students, and they immediately understood what Ian meant.

"That is to say, 1, 3, 5, 7, and 9 are odd numbers, and 2, 4, 6, and 8 are even numbers."

"There is such a distinction in numbers." The little wizard with a pure-blood family was very surprised.

The Muggle-born wizard explained to him: "This is a classification and summary by Muggle mathematicians. This question asked by the knocker should also be a question about Muggle mathematics."

"What about 0? Is 0 an odd or even number?" Every Ravenclaw little wizard has the pursuit of knowledge, and soon the little wizards were aroused by mathematical knowledge.

Without Ian's answer, a clever little wizard deduced it.

"0 should be an even number, because there is no any number that satisfies 2n+1=0, and 0÷2=0 is divisible, so 0 should be an even number."

There is actually a loophole in what he said. If you bring negative half into the formula, you can get 0. However, Ian did not remind them that it is not easy for them to understand odd and even numbers in a short period of time. What about negative numbers? Let's talk about it later, save the confusion.

"Five-digit numbers, how many of them contain three odd numbers and two even numbers? This range is too large. If you count each number one by one, even if you count them for three days, you won't be able to count them all."

"The knocker will not give us unsolvable problems." You Xiaoying shared calmly, "There must be an easy way to calculate this number in a short time!"

They had no class in the morning, so they were not in a hurry, gathered in front of the door, and began to think about an easy way to solve this problem.